Efficient and ecofriendly protocol for tetrahydropyranylation/ depyranylation of alcohols in the presence of tin(II) chloride dihydrate

Article abstract of DOI:10.1080/00397910601055123

A mild, efficient, and solvent-free protocol for tetrahydropyranylation of alcohols in the presence of a catalytic amount of SnCl2·2H2O is reported. Simple filtration of the reaction mixture through a short silica-gel pad gives the pure products in excellent yields. Depyranylation can also be achieved by adding methanol under similar reaction conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910601055123

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Efficient and Ecofriendly Protocol for
Tetrahydropyranylation/Depyranylation of
Alcohols in the Presence of Tin(II) Chloride
Dihydrate
Dipankoj Gogoi a , Nabajyoti Baruah a & Ghanashyam Bez a
a Department of Chemistry , Dibrugarh University , Dibrugarh, Assam, India
Published online: 06 Mar 2007.
To cite this article: Dipankoj Gogoi , Nabajyoti Baruah & Ghanashyam Bez (2007) Efficient and Ecofriendly
Protocol for Tetrahydropyranylation/Depyranylation of Alcohols in the Presence of Tin(II) Chloride Dihydrate,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
37:4, 593-597, DOI: 10.1080/00397910601055123
To link to this article: http://dx.doi.org/10.1080/00397910601055123
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Synthetic Communicationsw, 37: 593–597, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601055123
Efficient and Ecofriendly Protocol
for Tetrahydropyranylation/Depyranylation
of Alcohols in the Presence of Tin(II)
Chloride Dihydrate
Dipankoj Gogoi, Nabajyoti Baruah, and Ghanashyam Bez
Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India
Abstract: A mild, efficient, and solvent-free protocol for tetrahydropyranylation of
.
alcohols in the presence of a catalytic amount of SnCl2 2H2O is reported. Simple fil-
tration of the reaction mixture through a short silica-gel pad gives the pure products in
excellent yields. Depyranylation can also be achieved by adding methanol under
similar reaction conditions.
Keywords: alcohols, microwave, solvent-free, tetrahydropyranylation
The protection of alcohols as tetrahydropyran[1] derivatives is one of the most
sought after protocols in organic synthesis because of their easy introduction,
greater stability toward hydrides, and alkylating reagents such as alkyl
2
.
lithiums, Grignard reagents and so. Although BF3 Et2O is the most
commonly used catalyst for tetrahydropyranylation of alcohols, its sensitivity
toward moisture makes the synthetic chemists to look for better catalysts, as a
result of which various catalysts such as In(OTf)3,[3] p-TSA,[4] PPTS,[5]
ZrCl4,[6] I2,[7] LiBr,[8] InCl3 immobilized in ionic liquid,[9] acetonyltriphenyl
phosphonium bromide (ATPB),[10] aluminium chloride hexahydrate,[11]
[13]
TBATB,[12] CuSO4 5H2O,
bromodimethyl sulphonium bromide,[14] dia-
.
lkylimidazolium tetrachloroaluminates,[15] K-10 clay,[16] natural kaolinitic
clay,[17] EPZG,[18] Y-zeolite,[19] alumina-supported zinc chloride,[20] and
Nafion[21] were discovered to affect this transformation. Ironically, many of
Received July 12, 2006
Address correspondence to Ghanashyam Bez, Department of Chemistry, Dibrugarh
University, Dibrugarh, 786004 Assam, India. E-mail: ghanashyambez@yahoo.com
593

594
D. Gogoi, N. Baruah, and G. Bez
them suffer from difficulties such as longer reaction times, harsh reaction con-
ditions, incompatibility with acid-sensitive functional groups,[2,9,11,12]
removal of azeotropic mixture,[4,5] use of dehydrating reagents, involvement
of expensive and moisture-sensitive catalysts,[3,4,10,14,15] and higher catalyst
loading.[6,9,11,12,17,19–21]
Organic reactions under solvent-free conditions[22] have increasingly
become popular because they are more environmentally favorable.[23]
Moreover, recent years have witnessed phenomenal growth of microwave
(MW) irradiation[24] in organic transformations.
In pursuance of our recent interest on microwave-accelerated reactions
under solvent-free conditions,[25] we looked for a reagent system that would
work under solvent-free conditions to make tetrahydropyranylation more
environmentally favorable and clean, yet operationally simple and cost
effective. We envisaged that tin(II) chloride, an extremely mild, cheap,
easily available, and moisture-insensitive Lewis acid, might work as a
wonderful catalytic system under solvent-free conditions to effect the trans-
formation. In fact, when a ground mixture of n-octanol, 3,4-dihydro-2H-
.
pyran (1 equiv) and 5 mol % of SnCl2 2H2O is irradiated with microwaves
at 100 W for 4 min, complete tetrahydropyranylation takes place. In this com-
.
munication, applications of our methodology (Scheme 1) using SnCl2 2H2O
as an efficient catalyst for tetrahydropyranylation/depyranylation of various
alcohols (Table 1) are reported.
Various alcohols (Entries 1–20, Table 1) gave excellent yields of their
corresponding THP ethers. Acid-sensitive functional groups such as acetoxy
(Entries 11, 17), isopropylidene (Entry 15), OTBS (Entry 18), and reducible
nitro group (Entry 3) are very much unaffected under this reaction condition
as evident from their percentage of yields. Phenolic hydroxy groups
(Entries 4, 14) remain unaffected under these conditions.[7] The allylic
alcohols, cyclohex-2-en-1-ol and cinnamyl alcohol (Entry 19, 20), also
generate their corresponding THP ethers in 93% and 91% yields respectively.
It has been observed that m-hydroxybenzyl alcohol, 4-hydroxy-3-methoxy
benzyl alcohol, gave only mono THP ethers, where phenolic hydroxy
groups remained inert under this reaction condition. To study the inertness
of phenols, when p-cresol (Entry 21) was treated with 3,4-dihydro-2H-pyran
under similar reaction conditions, no THP ether formation was observed.
Interestingly, THP ether of benzyl alcohol undergoes cleavage upon
microwave irradiation at 100 W within 3 min in the presence of the same
catalyst (5 mol %) in methanol to give 89% yield (Scheme 2). It has been
observed that THP ethers of other alcohols (Entries 1–10, Table 2) were
Scheme 1.

Tetrahydropyranylation/Depyranylation of Alcohols
Table 1. Tetrahydropyranylation of alcohols via Scheme 1a
595
Time
(min)
Yieldb
(%)
Entry
Substrates
1
2
PhCH2OH
3
3
3
3
3
3
4
4
4
4
3
3
3
4
4
91
95
93
88
92
89
87
86
90
83
89
93
91
87
88
p-ClC6H4CH2OH
p-NO2C6H4CH2OH
m-OH C6H4CH2OH
p-OMeC6H4CH2OH
m-(CO2Me)C6H4CH2OH
n-Octanol
3
4
5
6
7
8
Cyclohexanol
Menthol
9
10
11
12
13
14
15
Cholesterol
m-AcO C6H4CH2OH
1-Phenyl ethanol
1-(p-Bromophenyl) ethanol
4-Hydroxy-3-methoxy benzyl alcohol
16
17
18
19
20
21
Diphenylmethanol
3
4
95
88
83
93
91
—
4-Aceoxy butan-1-ol
4-(t-Butyldimethylsilyloxoy) butan-1-ol
Cyclohex-2-en-1-ol
Cinnamyl alcohol
4
3
3
10
o-Cresol
aMicrowave irradiation was done with a Samsung microwave, model 1630 N.
bYield of the pure isolated product were characterized by 1H NMR and IR spectroscopy.
also cleaved under similar reaction conditions to generate their corresponding
alcohols in 80–91% yields.
.
In conclusion, we have established SnCl2 2H2O as an extremely effective
catalyst for tetrahydropyranylation and depyranylation of alcohols containing
acid labile and reducible functional groups such as acetoxy, nitro, OTBS,
methoxy, carbomethoxy, and 1,3-dioxolane, which may find wide applicability
in multistep synthesis. In contrast to some of the previously reported methods,
our procedure is fast, is atom economic, and does not require an inert atmos-
phere, dehydrating agent, catalytic support, and aqueous workup. Simple fil-
tration through a short silica-gel pad gave the pure products in very good yields.
Scheme 2.

596
D. Gogoi, N. Baruah, and G. Bez
Table 2. Depyranylation of THP ethers via Scheme 1
Time
(min)
Yielda
(%)
Entry
Products
1
2
C6H4CH2OH
n-Octanol
3
3
4
4
3
3
4
4
3
3
89
81
91
82
82
86
81
80
82
87
3
p-NO2C6H4CH2OH
Cyclohexanol
4
5
m-AcO C6H4CH2OH
1-(p-Bromophenyl) ethanol
4-Aceoxy butan-1-ol
4-(t-Butyldimethylsilyloxoy) butan-1-ol
Cyclohex-2-en-1-ol
6
7
8
9
10
Cinnamyl alcohol
aYield of the pure isolated product was compared with starting alcohols and
1
confirmed by H NMR.
TYPICAL PROCEDURE
For tetrahydropyranylation, typically a mixture of n-octanol (0.260 g,
.
2 mmol), 3,4-dihydro-2H-pyran (0.168 g, 2 mmol), and SnCl2 2H2O
(0.023 g, 0.1 mmol) was taken in an Erlen-Meyer flask and irradiated with
microwaves at 100 W for 4 min. The reaction mixture was allowed to cool
and filtered through a small silica-gel (60–120 mesh) pad with 1% ethyl
acetate in hexane. Yield ¼ 87% (0.372 g, 1.74 mmol). IR(KBr): 1040, 1075,
1128, 1360, 1469, 2860, 2921 cm21
. d 0.85
1H NMR (CDCl3):
(t, J ¼ 6.5 Hz, 3H), 1.25–1.35 (m, 10H), 1.42–1.77 (m, 8H), 3.35–3.41
(m, 1H), 3.45–3.50 (m, 1H), 3.63–3.71 (m, 1H), 3.79–3.86 (m, 1H), 4.57
(m, 1H).
For dehydropyranylation, a solution of THP ether of benzyl alcohol
(0.384 g, 2 mmol) in methanol (2 mL) in a Erlen-Meyer flask was irradiated
.
with microwaves with SnCl2 2H2O (0.023 g, 0.1 mmol) for 3 min, and the
resulting solution was passed through a silica-gel column taking 10% ethyl
acetate in hexane as eluent to obtain the desired benzyl alcohol in 89%
(0.192 g, 1.78 mmol) yields.
ACKNOWLEDGMENTS
Authors are grateful to the vice-chancellor, Dibrugarh University, for giving
the facility to work. Thanks are also due to Nabin C. Baruah, Regional
Research Laboratory, Jorhat, and P.K. Gogoi, Chemistry Department, D.U.,
for their valuable suggestions and help.

Tetrahydropyranylation/Depyranylation of Alcohols
597
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